Referring to FIG. 5, there is shown an explanatory view showing an operating principle of a conventional redox flow secondary battery. The redox flow battery has a cell 100 separated into a positive electrode cell 100A and a negative electrode cell 100B by a membrane 103 that can allow ions to pass through. The positive electrode cell 100A and the negative electrode cell 100B include a positive electrode 104 and a negative electrode 105, respectively. A positive electrode tank 101 for feeding and discharging positive electrolytic solution to and from the positive electrode cell 100A is connected to the positive electrode cell 100A through conduit pipes 106, 107. Similarly, a negative electrode tank 102 for feeding and discharging negative electrolytic solution to and from the negative electrode cell 100B is connected to the negative electrode cell 100B through conduit pipes 109, 110. Aqueous solution containing ions that change in valence, such as vanadium ion, is used for the positive and negative electrolytes. The electrolyte containing the ions is circulated by using pumps 108, 111, to charge and discharge with the change in ionic valence at the positive and negative electrodes 104, 105.
Referring to FIG. 6, there is shown a diagrammatic illustration of construction of a cell stack used for the redox flow battery mentioned above. This type of battery usually uses the construction which is called a cell stack 200 comprising a plurality of cells stacked in layers. Each cell comprises the positive electrode 104 made of carbon felt and the negative electrode 105 made of carbon felt arranged at both sides of the membrane 103. Cell frames 210 are arranged at the outside of the positive electrode 104 and at the outside of the negative electrode 105, respectively.
Each of the cell frames 210 comprises frames 212 made of plastic and a bipolar plate 211 made of a plastic carbon fixed in between the frames 212. The positive electrode 104 and the negative electrode 105 are adhesively bonded to the bipolar plate 211.
End plates 201 are arranged at both sides of the stack body comprising the cell frames 210 and the electrodes 104, 105 and are clamped onto the both sides of the stack body by tightening nuts 203 screwed on rod-like members 202 piercing both end plates 201. The end plate 201 commonly used comprises a rectangular plate 201A reinforced by a latticed frame 201 B integrally formed on the rectangular plate 201 A.
However, the conventional cell stack 200 involves the adhesively bonding of the positive electrode 104 and the negative electrode 105 to the bipolar plate 211, leading to increase in the fabrication process.
In addition, the bonding of the bipolar plate 211 to the electrodes 104, 105 by adhesive involves the disadvantage that due to deterioration of the adhesive, there is the possibility that the electrodes 104, 105 may peel off from the bipolar plate 211. This results in increase in electrical internal resistance of the battery, providing the problem of causing reduction of the battery efficiencies.
Accordingly, it is a primary object of the present invention to provide a cell stack for a redox flow battery that can maintain its reliability over a long term without adhesively bonding the bipolar plate to the electrodes.